TAPS can be diagnosed 2 ways – antenatally and postnatally.
Antenatal TAPS is diagnosed using Doppler measurements of the middle cerebral artery peak systolic velocity (MCA-PSV Doppler). This is an ultrasound to see how fast blood is flowing through each baby’s brain. Ideally, these should start between 16 and 20 weeks in your pregnancy .
The MCA-PSV Doppler measures the speed blood is going through the brains of your babies. This speed is measured in centimetres per second, and later converted into a Multiple of the Median (MoM), which is a fancy way of saying it’s measured against the average speed for the gestational age of the babies. These points are plotted on a curve graphed from the median .
In an ideal world, this MoM number is 1, or is the average, with both babies’ Doppler readings close together .
However, TAPS is suspected when one baby’s blood is flowing too fast, and the other too slowly. The anaemic twin’s blood is moving quickly through the brain, and the polycythemic twin’s thick blood is moving very slowly. As the measurements move apart, this is a good indication of the stage of TAPS they are in .
The following staging represents the recently improved TAPS classification system by Tollenaar et al. .
Stage 1: Delta MCA-PSV > 0.5 MoM, without signs of fetal compromise
Stage 2: Delta MCA-PSV > 0.7 MoM, without signs of fetal compromise
Stage 3: As Stage 1 or 2, with cardiac compromise of donor *
Stage 4: Hydrops of donor
Stage 5: Intrauterine demise of one or both fetuses preceded by TAPS
* Defined as critically abnormal flow: Doppler shows absent or reversed end-diastolic flow in umbilical artery, pulsatile flow in umbilical vein and/or increased pulsatility index or reversed flow in ductus venosus.
In the above description, “Delta” 0.5 and 0.7 are referring to MoM readings that have a difference larger than 0.5 or 0.7. For instance, MoM readings of 0.9 in one twin and 1.6 in the other twin could indicate Stage 2 TAPS, because those readings are different by 0.7.
Routine MCA-PSV Dopplers are the only way to detect antenatal TAPS [13, 17]. Knowing if your babies have TAPS is crucial because you will need interventions in order to help them. Simply put, if you are not having MCA-PSV Dopplers performed, you will not know that they have TAPS, and if you do not know that they have TAPS, you cannot provide them with life-saving interventions .
Postnatal TAPS is diagnosed via blood testing and placental pathology. Your babies will need their reticulocytes and haemoglobin tested, as well as their placenta sent for pathology to be checked for tiny connections between the babies .
They will calculate the difference in numbers between the babies, with first subtracting the donor’s haemoglobin count from the recipients. They will also divide the donor’s reticulocyte count by the recipient’s, and calculate the ratio of these in the readings. If there is a difference of more than 8.0 in the haemoglobin, and reticulocyte ratios are above 1:1.7, then this indicates that TAPS was present.